removed unused using directive that's causing build issues

Author: CreepGin

Runtime/CustomStyleSheets/unity-6.3+/StyleSheetImportErrors.cs

@@ -1,7 +1,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
-using UnityEditor.UIElements.StyleSheets;
 using UnityEngine;
 
 namespace OneJS.CustomStyleSheets {